Trace metal monitoring has become increasingly important recently in connection with efforts to avoid environmental pollution, and there now exists legislation prescribing extremely low maximum tolerable concentrations of, for example, less than one part per million (p.p.m.) for certain metals such as Hg, Pb, Cd and As in waste streams.
The Environmental Protection Agency has recognized atomic absorption analysis as a preferred technique for most trace metals because of its inherent excellent sensitivity and selectivity capabilities when compared with titration, colorimetry and ion selective electrode techniques. However, conventional atomic absorption techniques do not provide the resolution required to distinguish the presence of trace metals from those matrices which produce strong background absorption signals.
Other shortcomings in conventional atomic absorption spectrometry include a failure to recognize fully the significance of lamp drift and furnace-generated noise in their effects on measurement accuracy. Specifically, means such as flame atomizers have been resorted to with various beam chopping devices to limit the introduction of noise into the measurement signal. However, when an analysis is completed within a short time span, e.g., of the order of two seconds, chopping rates must necessarily be very high in order to avoid the noise problem.
Moreover, standard dual beam atomic absorption analytical techniques do not direct the reference beam through the sample, but, instead, pass the reference beam around it. This precludes correction for background absorption in the measurement signal. In those conventional systems which use a second light source for background absorption correction by passing radiation through the sample, lamp drift correction is precluded. In addition, those spectrometers employing two lamps, two detectors and two synchronized choppers will experience the effects of photomultiplier (PM) tube drift unless the analysis is performed in a short time span. Finally, previous Zeeman effect systems have relied upon complex or fragile devices such as mechanical or electro-optical beam choppers or expensive single purpose filters, such as the atomic vapor filter described by Hadieshi and McLaughlin (ref: Science Oct. 22, 1971) to separate the two beams.
Successful application of atomic absorption analysis to industrial service requires simplicity of control (or, preferably, automation) to permit use by relatively unskilled personnel, system ruggedness to withstand long term service in demanding environments, measurement repeatability in order to detect immediately the onset of effluent system malfunctions, flexibility in the accommodation of product line changes and low maintenance requirements. It is the object of this invention to provide an improved Zeeman effect atomic absorption spectrometer which solves all of the foregoing problems.
The Zeeman effect is the phenomenon that, when an emitting atomic mass in the atomized (i.e., atomic vapor) state is placed in a magnetic field, the emitted radiation, in a direction normal to the magnetic field, propagates as two distinctive linearly but orthogonally polarized beams of equal intensity. One beam, hereinafter denoted the "analytical beam," polarized parallel with the magnetic field, contains a wavelength, .lambda..sub..pi., which is nearly identical with the wavelength of the emitting source in the absence of the magnetic field. The other beam, hereinafter denoted the "reference beam", is polarized orthogonally with respect to the analytical beam and consists of radiation of at least two different wavelengths, .lambda..sub..sigma..sub.+ and .lambda..sub..sigma..sub.-, which lie equally spaced on either side of the central .lambda..sub..pi. radiation. The separation in wavelength between the reference and analytical beams is very small, being only on the order of a hundredth of an angstrom unit (10.sup..sup.-8 cm.), dependent on the strength of the magnetic field. Thus, if one chooses a wavelength of the analytical beam to coincide with the characteristic spectral absorption line of a trace element in the atomized state which it is desired to analyze, the other (reference) beam can effectively monitor the substantially wavelength-independent background absorption characteristic of the analyte.
In order to reduce noise which interferes with the conduct of analysis, this invention resorts to acyclic beam generation and processing, in that it utilizes substantially steady state D-C magnetic field application to the radiation emitter and thereafter employs continuous form analytical and reference radiation beams. The beams are separated spatially by a polarizing prism which is provided with a monochromator or a selected narrow band pass optical thin film filter to block off extraneous background radiation, after which the intensities are separately transduced to electrical analog signals and the logarithmic ratio taken to nullify the effects of strong background absorption and source intensity drift on the trace element mass measurement.
Optional features improving operation constitute a long lifetime resistively heated metallic tubular furnace, a digital reference signal storage means to reduce the need for repeated calibrations, automatic sequencing of programmable analysis steps, and collocation of the monochromator and the polarizing prism to simplify instrument alignment.
Measurement repeatability is enhanced by the use of non-reactive tubing in the sampling system together with complete purging of the microsampler itself to safeguard against sample contamination. A base line clamping circuit is used to nullify slow drifts in the P.M. tubes and preceding circuitry, whereas the ratioing of the dual beams compensates for spectral line source base line drift.